PROJECT 1: PROJECT SUMMARY Thrombocytopenia is, after anemia, the most common hematological problem among sick neonates, affecting 20-35% of NICU patients. In the U.S., platelet transfusions are frequently given to neonates with mild to moderate thrombocytopenia (platelet counts 50-100x109/L). A substantial percentage of these transfusions (58% in our recent multicenter retrospective study) are given to neonates with inflammatory conditions, such as sepsis or necrotizing enterocolitis (NEC), usually when the platelet count falls below an arbitrary and highly variable threshold. However, a growing body of evidence supports the fact that platelet counts are poor predictors of bleeding. Furthermore, it has been recently recognized that platelets play important roles in inflammation and host defense, and our preliminary data suggest that adult platelets are more pro- inflammatory than neonatal platelets. Currently, neonatal transfusion decisions are made without taking into account the differences that exist between neonatal and adult platelets, or the potential consequences of thrombocytopenia and platelet transfusions on neonatal inflammation or host defense against bacterial organisms. Our long term goal is to improve the treatment of thrombocytopenic neonates through a better understanding of the systemic effects of thrombocytopenia and platelet transfusions. Our central hypothesis is that platelet transfusions will be beneficial to some neonates due to their hemostatic functions and roles in host defense, but will be harmful to others by exacerbating ongoing inflammation. To test these hypotheses, we have formulated the following Specific Aims: 1) To define the effects of thrombocytopenia and platelet transfusions on neonatal inflammation; 2) To establish the impact of neonatal thrombocytopenia and platelet transfusions on bacterial sepsis; and 3) To characterize the effects of neonatal thrombocytopenia and platelet transfusions on markers of systemic inflammation and clinical bleeding in human neonates. These aims will be accomplished using a combination of translational studies applying our published miniaturized human in vitro platelet transfusion system to evaluate the effects of adult platelets on neonatal blood (S.A. 1 and 2), murine models of neonatal anemia, inflammation, sepsis, and platelet transfusions (S.A. 1 and 2), and clinical studies of human thrombocytopenic neonates (S.A. 3). The studies in Specific Aims 1 and 2 will provide mechanistic insights into the effects of thrombocytopenia and platelet transfusions on neonatal inflammatory responses, and on bacterial sepsis. Specific Aim 3 will systematically assess the potential beneficial effects of platelet transfusions on clinical bleeding (utilizing a validated neonatal bleeding score) vs. their potential negative effects inducing NET formation and amplifying inflammation. These studies will -for the first time- consider the role of platelets as inflammatory modulators in the context of neonatal transfusion medicine. We anticipate that our findings will establish a basis for a more individualized, physiology-driven approach to neonatal platelet transfusions.